Chris,

Another troubleshooting technique to discern between hardware and software is to populate the board a section at a time. For example, since you're building a calculator I'll assume you've got a keyboard and a display. Start with the keyboard - connect it to the board, then begin pressing keys and look for the lines going into the microprocessor socket changing state when the appropriate key is pressed. If you've got a 4x4 keyboard that needs to be scanned, alter the test procedure to test just the keyboard connections. Now, I know you've got a display since you mention a 7447, further I know the micro will not convert hex or binary to seven segment drive since that's what the 7447 does. So, put the 7447 and display in place and apply power to that section of the circuit. Stimulate each 7447 with a four bit code (0-F, hex). Does each display show a pattern associated with the appropriate four bit code? If so, go to the next portion of the hardware.

Then move to the code portion of the project. Eric, Peter and Craig have given great suggestions and the common theme is write a small bit of code and test it.

Have Fun,

Bruce